1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for dimmer compatibility with loads that include a reactive impedance.
2. Description of the Related Art
The power control systems often utilize dimmers to establish an amount of power to be delivered to a load such as one or more light emitting diodes or other electronic devices. A typical dimmer is inserted in a circuit in series with a supply voltage source and a load. The dimmer phase cuts the supply voltage, which reduces the average power delivered to the load. The degree of the phase cut can be changed, such as with a user input, which, thus, allows the dimmer to modulate power delivered to the load. Modulating the power delivered to the load facilitates a number of common functions, such as dimming a lamp to reduce the brightness of the lamp.
FIG. 1 depicts a lighting system 100 that includes a dimmer 102 that modulates power delivered to an incandescent lamp 104. FIG. 1A depicts exemplary signals for the lighting system 100 when the dimmer 102 is in a dimming mode. Referring to FIGS. 1 and 1A, voltage source 106 supplies an alternating current (AC) input voltage VSUPPLY for the lighting system 100. Voltage waveform 150 represents one embodiment of the supply voltage VSUPPLY. The voltage source 106 is, for example, a public utility, and the AC voltage VSUPPLY is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The dimmer 102 is connected in series with the lamp 104, and the voltage source 106 so that a supply current iSUPPLY flows from the voltage source 106, the dimmer 102, and the lamp 104.
Dimmer 102 is commonly referred to as a “smart dimmer”. Smart dimmers are generally referred to as a class of dimmers that include a controller, such as controller 110. The dimmer 102 includes a user interface 108 that, for example, receives dimming level inputs from a user. A controller 110 is connected to a communication circuit 112 to, for example, transmit and receive control information via the transformer 114 to and from other dimmers (not shown) that may also be connected to the voltage source 106. The dimmer 102 also includes a power supply 116 that includes a triac-based phase cutter 118 and a charging circuit 120. The controller 110 controls the phase cut angle of the supply voltage VSUPPLY, and the phase cut supply voltage is supplied as a dimmer voltage VDIM. Voltage waveform 151 depicts an exemplary cycle of dimmer voltage VDIM with phase cuts 152 and 154. During the period 155, which occurs once per cycle of the supply voltage VSUPPLY during the phase cut portion of the dimmer voltage VDIM, the dimmer 102 essentially functions as a current source to supply a dimmer current iDIM during period 155 to the charging circuit 120. The charging circuit 120 utilizes the current iDIM during the period 155 to generate direct current (DC) voltage VCC. Voltage VCC provides an operational supply voltage to the user interface 108, controller 110, and communication circuit 112. U.S. Pat. No. 7,423,413 contains a more detailed, exemplary description of lighting system 100.
FIG. 2 depicts exemplary signals 170 when dimmer 102 is in an OFF mode. Referring to FIGS. 1 and 2, when the dimmer 102 is in an OFF mode, the dimmer voltage VDIM is approximately 0V, and the lamp 104 is not drawing any power. However, the charging circuit 120 continues to draw current during the portion 155 of each cycle of the supply voltage VSUPPLY. Since the lamp 104 has a resistive impedance, the current iDIM continues to flow during the OFF mode. The charging circuit 120 utilizes the reduced supply current iSUPPLY to maintain voltage VCC during the OFF mode so that the dimmer 102 can continue to operate and respond to user commands via the user interface 108. The lighting system 100 is a 2-wire system that has a “hot” wire 126 and a ground wire 126. Governmental regulations often prevent current return directly from the dimmer 102 to earth ground. However, the lamp 104 provides a low resistance current return path for the supply current iSUPPLY even during the OFF mode.
FIG. 3 depicts a lighting system 300 that includes the dimmer 102 coupled to a reactive load 302 via full-bridge diode rectifier 304. A “reactive” load is a load that includes capacitive and/or inductive impedances. The load 302 in lighting system 300 includes a capacitor 308, which has a reactive impedance, and includes an LED lamp 306. LED lamp 306 operates with a DC output voltage VOUT and is dimmed by changing the DC level of the output current iOUT. To provide the DC output voltage VOUT, the rectifier 304 rectifies the dimmer voltage VDIM to generate the rectified dimmer voltage Vφ—R. Capacitor 308 filters any high frequency signals from the rectified dimmer voltage Vφ—R. The capacitance value of capacitor 308 is a matter of design choice. In at least one embodiment, the capacitance value is sufficient to reduce electromagnetic interference to acceptable levels and is, for example, 22 nF. The switching power converter 310 converts the rectified dimmer voltage Vφ—R into the DC output voltage. Controller 312 controls the switching power converter 310 to generate a desired level of the DC output voltage and output current iOUT and provide power factor correction. The switching power converter 310 can be any type of switching power converter such as a boost, buck, boost-buck, or Ca switching power converter. Prodić, Compensator Design and Stability Assessment for Fast Voltage Loops of Power Factor Correction Rectifiers, IEEE Transactions on Power Electronics, Vol. 22, No. 5, September 3007, pp. 1719-1729, describes an example of controller 312.
Referring to FIGS. 2 and 3, when the dimmer 102 is in the OFF mode, the capacitor 308 will eventually discharge, and the dimmer 102 will no longer have a source of charge for the power supply 116 to maintain the voltage VCC. The charging circuit 120 (FIG. 1) of dimmer 102 still needs a supply current iSUPPLY to generate the voltage VCC so that dimmer 102 does not lose internal power. However, the level of the dimmer voltage Vφ—R across capacitor 308 during the OFF mode will be greater than or equal to the dimmer voltage VDIM. Thus, the rectified dimmer voltage Vφ—R across capacitor 308 prevents the dimmer current iDIM from flowing through the power supply 116. Accordingly, without the dimmer current iDIM, the dimmer 102 will lose internal functionality during at least the OFF mode.